The high proclivity of essentially pure silver objects to tarnish in an atmosphere containing only traces of hydrogen sulfide has resulted in numerous alloys which are reputed to be tarnish-resistant. Such alloys include 92.5% silver, 0.5% copper, 4.25% zinc, 0.02% indium, 0.48% tin, and 1.25% boron-copper alloy containing 2% boron and 98% copper disclosed in U.S. Pat. No. 4,973,446; a silver-palladium alloy containing 80-92.5% silver, 4-9% palladium, 0-10% copper and 0.5-1% indium or zinc disclosed in U.S. Pat. No. 5,037,708; an alloy containing 90-92.5% silver, 5.75-7.5% zinc, 0.25-1% copper, 0.25-0.5% nickel, 0.1-0.25% silicon, and 0-0.5% indium disclosed in U.S. Pat. No. 5,817,195; an alloy containing 90-94% silver, 3.5-7.35% zinc, 1-3% copper, and 0.1-0.25% silicon, disclosed in U.S. Pat. No. 5,882,441; an alloy containing at least 99.5% silver with the balance chosen from a readily oxidizable element such as aluminum, antimony, etc. disclosed in U.S. Pat. No. 6,139,652; and several other alloys all of which purport to be tarnish-resistant but fail to provide any evidence of such resistance, particularly in an atmosphere containing a trace quantity of hydrogen sulfide and/or sulfur dioxide.
The term “tarnish” refers to the dulling of the luster of a metallic silver surface progressing to shades of grey which intensify over time. More specifically “tarnish” refers to “a reaction product that occurs readily at room temperature between metallic silver and sulfur in any form. The well-known black film that appears on silver, results from reaction between atmospheric sulfur dioxide and metallic silver forming silver sulfide. The film is easily removed from the surface with a cleaning compound and is not a true form of corrosion.” (See Hawleys's Condensed Chemical Dictionary, 14th edition, 2001 John Wiley & Sons, New York). As shown below, sulfur dioxide per se, does not cause tarnishing of silver. Tarnishing is caused by hydrogen sulfide in the presence of oxygen and moisture, and the tarnishing may be aggravated by sulfur dioxide.
As a result of having to counteract the tarnishing of silver objects, made from either essentially pure, that is at least 99.5% pure silver, or from one of such alloys, a large variety of organic compounds have been developed to clean the tarnished surfaces. A recent such example is disclosed in U.S. Pat. No. 6,165,284.
U.S. Pat. No. 4,944,916 to Franey discloses mixing particles of copper and aluminum in a polymer to scavenge permeating gases and, in an illustrative example, tested the effect on a silver lead frame, sealed in a bag, and placed in a chamber which was evacuated and back-filled with pure hydrogen sulfide. Pure hydrogen sulfide in the absence of moisture and molecular oxygen produces no tarnishing on an essentially pure lustrous silver surface.
Box-like containers and synthetic resinous film of polyethylene (“PE”), polypropylene (“PP”) and biodegradable polyester are most commonly used to package silver objects to protect them against contact with moisture and atmospheric pollutants; most preferred are transparent packages, and most preferably the polymer is low density polyethylene (“LDPE”). LDPE typically has a density less than 0.943 g/cc. By “transparent” is meant that the TI-polymer is to have substantially the same permeability to visible light as polymer which does not contain the TI ingredient(s). Such transparency allows one to view the silver object in the package and read this text in 12 point font with a piece of film laid over the text. The term “film” refers to polymer having a wall thickness sufficient to have substantially uniformly dispersed within it, enough of the TI ingredient to provide protection for a predetermined period. Further, the TI-polymer is to have smooth surfaces and a thickness which is substantially uniform, that is, its surfaces are smooth to the touch; e.g. smooth film typically has less than a ±0.0125 mm (0.5 mil or 0.0005″) variance in thickness, when the thickness of the film is in the range from 0.025 mm (1 mil or 0.001″) to 0.125 mm (5 mil or 0.005″) thick; when such variances are due to particles of solid, or clumps of particles, at or near the surfaces of the film, the film is not smooth to the touch.
The terms “silver” and “silver objects” are used herein to refer to objects made of a predominantly silver alloy typically containing in excess of 90% silver, such as is used to make jewelry, cutlery commonly referred to as “sterling silver”, cast figurines, busts or statues and other decorative furnishings; and, to refer to objects made of essentially pure silver, e.g. wiring in circuits of electronic devices, and jewelry, cutlery and similar objects referred to above, made of at least 99.5% silver. Though references which teach such alloys state that they are “tarnish resistant”, the references are conspicuously free of any reference to, or any evidence of, the resistance of such alloys to the action of sulfur dioxide and/or hydrogen sulfide. Permeation of gases through the walls of a package continues throughout the time the silver is stored, unless the tarnish-causing gases are intercepted and scavenged. Though tests indicate that damage due to sulfur dioxide, per se, contributes essentially nothing to the tarnishing of silver, to the extent that its presence may exacerbate the damage due to hydrogen sulfide, it is desirable to scavenge both gases.
Coating silver, alloyed or not, with a coating impermeable to the gases is often not an option, because or one reason or the other, the silver cannot be coated with an invisible protective coating such as a clear lacquer. In some instances, the presence of the lacquer is objectionable even if the cost of coating is not; in others, the lacquer-coated object must be shipped or otherwise delivered after a long time, at least one year, looking as if they had been freshly manufactured, by which time the lacquer coating becomes discolored, even if the silver surface beneath is not.
The silver objects to be stored are therefore sealed in packages the walls of which are relatively permeable to trace quantities of hydrogen sulfide in an amount in the range as low as from 10 ppb (parts per billion) up to 5 ppm (parts per million) or more. Thus the expense of packaging the silver, alloyed or not, so that it is not susceptible to tarnishing over a long time, is justified.
Because atmospheric oxygen, and oxygen-containing compounds such as nitrogen oxides as well as sulfur dioxide and hydrogen sulfide typically damage packaging film, it is routinely protected with an aromatic triazole, or, a hindered phenol antioxidant such as a 2,4,6-tri-substituted phenol having a 4-substituent selected from a substitutable methylene carbon and a substitutable amine. The triazole and/or phenol provide protection against oxidation of the film when used in a relatively low concentration, typically less than 100 ppm, to scavenge free radicals generated by oxygen or sulfur atoms entering the film; it is believed that presence of the triazole and phenol facilitates the trapping of water molecules in synthetic resinous films. The phenol functions by generating a host of compounds as a result of free radicals generated by the action of the oxidizing agent, but oxygen is not scavenged in the film and neither is hydrogen sulfide or sulfur dioxide.
It is evident that a scavenger for hydrogen sulfide and sulfur dioxide in an article of arbitrary shape and cross-section, should provide protection against tarnishing by functioning as an active TI ingredient. By “active” is meant that the ingredient interacts with both, the hydrogen sulfide and any pollutant acid gas, particularly sulfur dioxide. The article is typically a synthetic resinous (hereafter also “plastic”) film, or a box-shaped receptacle; in addition to silver objects, the film is used to protect electronic products having metallic silver circuits, and silver connectors packaged for storage and transportation; the box-shaped receptacle may be lined with cloth in which the silver is bedded. For shipment, metallic silver objects are commonly packaged in either a polyolefinic (“PO”) bag or container thermoformed of PO; or, in a relatively large container into which the objects, wrapped in TI-containing film, are placed. Dispersed in the film, as an integral part thereof, is the active TI ingredient.
No packaging for substantially silver objects has been disclosed to maintain a lustrous silver surface free of tarnish for a period of at least one year at ambient temperature of about 23° C. in an atmosphere containing at least 10 ppb to 10 ppm of hydrogen sulfide and/or sulfur dioxide, individually or together.